Insulin insufficiency related to pancreatic fibrosis and -cell dysfunction is present in almost every cystic fibrosis (CF) patient. Progressive abnormalities in insulin secretion begin in childhood, and, in adults, CF related diabetes (CFRD) is eventually present in more than half of the CF population. CFRD is associated with weight loss, protein catabolism, loss of lean body mass (LBM), and early death from lung disease and malnutrition. The negative consequences of diabetes are just the tip of the iceberg, since clinical deterioration has been documented to begin in the pre-diabetic period. Non-diabetic glucose tolerance abnormalities in CF are associated with protein catabolism, weight loss and lung function decline, all of which correlate with the severity of insulin secretory defects, suggesting a key pathologic role for insulin insufficiency. Insulin is a potent anabolic hormone, critical for maintenance of body weight and muscle mass. In a placebo-controlled clinical trial, insulin therapy improved body mass index (BMI) and LBM in patients with very early CFRD (CFRD without fasting hyperglycemia), and this is now standard care for these patients. There is growing preliminary evidence that insulin therapy is beneficial even earlier, in CF patients with pre-diabetes due to insulin insufficiency. Given the universal prevalence of insulin insufficiency in CF, the high lifetime risk of developing diabetes, the clinical impact of insulin insufficiency n protein catabolism and survival in CF, and the critical importance of maintaining body weight and LBM in this population, there is an urgent need to determine whether insulin replacement therapy should be instituted for anabolic purposes prior to the actual onset of diabetes and, if so, to ascertain the optimal regimen. The current protocol describes a double-blind, placebo-controlled trial to determine whether insulin therapy improves protein catabolism in youth with CF and abnormal glucose tolerance, and to explore differences in efficacy between multiple daily pre-meal insulin dosing (as is currently standard for early CFRD) versus a more convenient once daily basal insulin dose (as has been used in small uncontrolled pilot studies). The findings of this study will provide a mechanistic rationale for instituting insulin in youth wih CF and pre-diabetes, and will inform both research studies and clinical practice as to the best regimen for insulin delivery in this population.